But according to website Aviation Wales. the vapours were left behind by F15 Strike Eagles which regularly train in the skies over the region.

They do not appear on flight-tracking websites as it is a security issue.

Air chemist Dr Gavin Phillips, who is senior lecturer in the Department of Natural Sciences at the University of Chester, said: “The long thin clouds, or contrails, that form behind the flight path of aircraft are caused by water vapour in the jet engine exhaust condensing as ice in very cold air.

“Sometimes the water vapour can condense onto particles of pollution emitted from the jet engine.

“They have been known to persist for many hours.

“When I lived in Alaska, and when it was cold enough, I saw the cars leave contrails.